Reprocessed records of seafloor vertical motion prior to the 2011 Tohoku earthquake

Ocean-bottom pressure records obtained near the epicenter of the 2011 Tohoku-Oki earthquake were examined to test whether the earthquake was preceded by substantial precursory crustal deformation. The seafloor data enabled us to search for small-scale preslip near the epicenter that would be difficult to identify from terrestrial geodetic data. After removing ocean-tide variations and nontidal fluctuations predicted by a global ocean model, the seafloor level records still contained noise originating from nontectonic processes. Taking advantage of the strong similarity of the noise among the different observing stations, we used principal component analysis to extract part of this noise component from the records. The resulting seafloor level data enabled us to detect deformations of 2-4 cm. This time series showed no significant preseismic crustal deformation other than gradual deformation due to afterslip of the Mw 7.3 foreshock of 9 March. Although our results did not exclude the possibility of preslip, they suggest that the size of any slip was no larger than the equivalent of a Mw 6.2 event, if it occurred near the hypocenter associated with the rupture nucleation process, or no larger than Mw 6.0 at the trenchward end of the subduction interface.